Fix warnings: ex_getln.c: init_history(): Nonnull attr passed null: HI.

Problem    : Argument with 'nonnull' attribute passed null @ 4227.
Diagnostic : Harmless issue.
Rationale  : It's true we're calling memset with NULL pointer, but it's
             also true we're doing it with zero size. We just thought
             that would work and do nothing (it was a way of avoiding
             to add a guard). It doesn't work, though, as memset
             requires nonnull arguments.
Resolution : Add guard to avoid null argument.

src/nvim/ex_getln.c

@@ -4239,7 +4239,9 @@ void init_history(void)
 
       // clear remaining space, if any
       int l3 = j < 0 ? 0 : MIN(newlen, oldlen);  // number of copied entries
-      memset(temp + l3, 0, (size_t)(newlen - l3) * sizeof(*temp));
+      if (newlen) {
+        memset(temp + l3, 0, (size_t)(newlen - l3) * sizeof(*temp));
+      }
 
       hisidx[type] = l3 - 1;
       xfree(history[type]);